DESCRIPTION: (Applicant's Abstract) This is the fourth renewal (but third from my present post at the University of Minnesota), of my Career Research Scientist Award (K05 DA70554-27) which releases me from all the classroom teaching and most of the administrative duties at the University. The overall goal is to continue my long-standing research program in the molecular mechanisms of opioid actions and in the molecular basis of opioid tolerance. The current proposal is to test the hypothesis that the alteration in mu-opioid receptor synthesis and regulation is related to the mechanism of morphine action and morphine tolerance. To achieve this goal, we plan to carry out two independent but related studies: 1) to determine the regulation of mu-opioid receptor gene expression which controls the synthesis of receptors in specific cells. We hypothesize that the difference in spatial and temporal expression of mu-opioid receptor gene are caused by (or resultant of) characteristic interactions between cis- and transregulatory elements that exist in different cells (tissues) at different times. Therefore, we plan to define these elements using in vitro and in vivo models, 2) to determine the molecular mechanism of mu-opioid receptor regulation by a chemical modification (i.e., phosphorylation) and determine its possible relationship to morphine tolerance. In these studies we aim to elucidate the exact role of receptor phosphorylation/ dephosphorylation on cellular adaptation of chronic opioid treatment (i.e., tolerance). We also plan to pinpoint the exact phosphorylation sites of cloned opioid receptors which are involved in receptor desensitization as well as to determine the protein kinases (or other protein factors) that are involved in this process.